A horizontal-axis wind turbine typically includes a tower, a nacelle supported by the tower, and a rotor mounted to the nacelle. Over time there has been a significant increase in the overall size of these machines and their components. This increase in size presents many challenges, both before and during operation.
For example, wind turbine towers are tall, slender structures typically comprised of cylindrical and/or conical sections. As wind blows across the tower, vortices are shed alternately from opposite sides of the tower. This gives rise to a fluctuating force acting substantially perpendicular to the wind direction. The fluctuating force can lead to large oscillations when the periodic frequency of the vortex shedding is similar to one of the natural frequencies of the tower.
When the nacelle is installed on a tower, vortex shedding from the wind alone (i.e., when the wind turbine is not in operation) is less of a significant concern. The fluctuating forces have a negligible effect because the massive weight of the nacelle changes the dynamic behavior of the overall structure. Prior to supporting the nacelle, however, the tower is more susceptible to oscillations from vortex shedding. As towers increase in size, lower wind speeds can produce vortex shedding in resonance with a natural frequency of the tower. Thus, large oscillations from vortex shedding become more of a concern.